1. Field of the Invention
The present invention relates to a method and apparatus for measuring a refractive index distribution of a test object such as an optical element.
2. Description of the Related Art
A high refractive index is required for an optical element such as a lens used for an optical unit, for example, a digital camera or a laser beam printer. An optical glass or plastic material having a high refractive index can easily be made into a complex shape of an aspheric surface or the like by using the molding technology.
However, molding may cause an uneven refractive index distribution in the optical element depending on molding conditions. The uneven internal refractive index distribution may greatly deteriorate an optical characteristic of the optical element, and a predetermined optical characteristic may not be maintained. Therefore, optical homogeneity in the optical element having a high refractive index needs to be measured with high accuracy.
A fringe measurement method is generally known as a method for measuring the optical homogeneity. The fringe measurement method highly accurately works a test object (optical element) and measures its transmission wavefront, thereby measuring its internal refractive index distribution. Another proposed method reduces a surface accuracy error of a test object by holding the test object between glass plates and by immersing the test object in oil that has a refractive index approximately equal to that of the test object and is poured between the glass plates.
Japanese Patent Laid-Open No. (“JP”) 01-316627 discloses a method for obtaining an optical characteristic of a test object by immersing the test object in a medium (matching oil) having a refractive index approximately equal to that of the test object to measure a transmission wavefront. This method can obtain the internal refractive index distribution of the test object without highly accurately working the test object.
A measurement method disclosed in JP 02-008726 measures a transmission wavefront by immersing a test object in first matching oil having a refractive index approximately equal to that of the test object. The method further measures a transmission wavefront by immersing the test object in second matching oil having a refractive index slightly different from that of the test object. The method obtains a shape and a refractive index distribution of the test object from a measurement result using the first matching oil and a measurement result using the second matching oil.
In the measurement using the second matching oil, influences of the refractive index distribution and the shape of the test object appear as an interference pattern in a detector. The refractive index of the second matching oil needs to be slightly different from that of the test object to the extent that interference patterns are not excessively dense.
The measurement methods disclosed in JPs 01-316627 and 02-008726 need matching oil having a refractive index approximately equal to that of the test object. However, the matching oil of a high refractive index has a low transmittance. Thus, the measurements of the transmission wavefront of the optical element of a high refractive index by the methods disclosed in JPs 01-316627 and 02-008726 can obtain only a weak signal from the detector, and their measurement accuracies are low.